Internal cooling with liquid gas

ABSTRACT

In a method for the internal cooling of a rotating object ( 4 ), liquid gas from at least one inlet channel ( 61 ) in a fixed object ( 5 ) is pressed into a ring-shaped groove ( 62 ) located between the fixed object ( 5 ) and the rotating object ( 4 ). From the ring-shaped groove ( 62 ) the liquid gas is pressed into at least one channel ( 63, 64.1, 64.2 ) in the rotating object ( 4 ) and brought to a part to be cooled ( 15 ). The liquid gas, upon contact with the part, evaporates and expands while absorbing heat, thereby cooling the part. The area surrounding the part to be cooled ( 15 ) may be designed as an expansion chamber ( 65 ). The method can be used to cool molded parts in injection molding machines with rotating molds ( 1 ), thereby achieving shorter cycle times.

BACKGROUND OF THE INVENTION

The invention relates to a method for the internal cooling of a rotatingobject with liquid gas and to a device for implementation of the coolingmethod. The method is suitable in particular for the cooling of moldedparts in injection molding machines with rotating molds.

In many fields of technology there is a requirement for an internalcooling of a rotating object. Such an internal cooling is achieved byintroducing a cooling liquid, for example water, from a non-rotating,fixed object into the at least partially adjoining rotating object. Inthe rotating object, the cooling liquid is then conveyed to the part tobe cooled. The liquid absorbs the thermal energy from the part and takesheat away, thereby producing a cooling effect. In doing so, the rotatingobject, for example, can be designed as a shaft, the fixed object, forexample, as a bearing for supporting the shaft.

The introduction of the cooling liquid from the fixed object into therotating object usually takes place axially. The transfer point betweenthe fixed and rotating objects is situated on the axis of rotation. Suchan arrangement is desirable, because with it the transfer point is notmoving relative to the fixed object. There are, however, instances,where the rotating object is not axially accessible.

In some applications it is desirable to cool with a liquid gas insteadof with a conventional liquid. Cooling processes with liquid gas areknown as such. In them, a liquid gas is conveyed to the part to becooled, whereby it usually is compressed all the more, the closer it isto the part to be cooled. At the part to be cooled an evaporation and anexpansion of the initially liquid, compressed gas is permitted. Duringevaporation and expansion, thermal energy is withdrawn from the part tobe cooled, as a result of which a cooling effect is produced. The gas isthen removed in a gaseous physical condition (condition of aggregation).

If such a liquid gas is to be introduced into a rotating object from afixed object, particular problems occur. The liquid gaspossibly-depending on its chemical composition, temperature andpressure-is in a special physical condition (condition of aggregation),which, while advantageous for the cooling, is exceedingly delicate withrespect to the handling. In any case, an evaporation and/or expansion ofthe liquid gas has to be avoided, because this would lead to freezing ofthe transfer point. Because of the special physical condition (conditionof aggregation) of the liquid gas, in particular sealing problems at thetransfer point have to be solved.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for the internalcooling of a rotating object with liquid gas, which solves or minimizesthe problems set forth hereinbefore. It is furthermore an object of theinvention to create a device for the implementation of this method.

In case of the method in accordance with the invention for the internalcooling of a rotating object, liquid gas is pressed from at least oneinlet channel in a fixed object into a ring-shaped groove disposed at acontact surface between the fixed object and the rotating object. Fromthe ring-shaped groove, the liquid gas is pressed into at least onechannel in the rotating object and fed to at least one part to be cooledat which point the liquid gas evaporates, absorbs vaporization heat, andis removed as gaseous gas.

In preference, in an area surrounding the part to be cooled a greatercross-sectional surface area is made available to the liquid gas forflowing through, such as the sum of the cross-sectional surface areas ofthe at least one channel. As a result of this greater cross-sectionalsurface area in the area surrounding the part, the liquid gas evaporatesand expands while absorbing thermal energy. The volume of the gasfollowing the expansion, for example, can amount to 600 times its volumeprior to the expansion.

The total cross-sectional surface area, which is made available to theliquid gas on the way from the fixed object to the object to be cooled,is preferably maintained constant or reduced, so that the liquid gasdoes not expand on the way to the part to be cooled and so that anoptimum cooling effect is obtained at the part to be cooled. It isparticularly advantageous to reduce the total cross-sectional surfacearea at least once, which is made available to the liquid gas on the wayto the part to be cooled, so that the liquid gas is compressed. This canbe achieved, for example, by contractions of the channels. While theinlet channel in the fixed object may have a diameter of severalmillimetres, the last section of the channel in the rotating object mayhave a diameter of 0.5 mm or less; even capillary dimensions can beutilized. In this regard, “total cross-sectional surface area” is: thecross-sectional surface area of the one channel, if only one channel ispresent, resp., the sum of the cross-sectional surface areas of allchannels, if several channel are present; in this, the cross-sectionalsurface areas are always measured vertical or perpendicular to thedirection of flow of the gas.

The device in accordance with the invention for the implementation ofthe method has a fixed object, in which an object rotating around arotation axis is rotatably fixed. The fixed object has at least oneinlet channel for liquid gas. The rotating object is surrounded by aring-shaped groove, into which the at least one inlet channel leads andthe center of which is situated on the rotation axis of the rotatingobject. The ringshaped groove can be machined into the rotating objectand/or into the fixed object. The rotating object has at least onechannel for liquid gas that leads out from the ringshaped groove intothe area surrounding a pan to be cooled.

The area surrounding the part to be cooled preferably has a greatercross-sectional surface area than the sum of the cross-sectional surfaceareas of the at least one channel in the rotating object, whereby thesecross-sectional surface areas are measured in essence vertically orperpendicular to the rotation axis. As a result of this, the gas isprovided with sufficient volume for an expansion. The area surroundingthe part to be cooled can, for example, be at least one expansionchamber.

The cross-sectional surface area of the inlet channel or, if severalinlet channels are present, the cross-sectional surface area of theinlet channels, is preferably greater than double the cross-sectionalsurface area of the ring-shaped groove. If this requirement isfulfilled, then the liquid gas does not evaporate and/or expand in thevicinity of the ring-shaped groove. Evaporation and/or expansion couldhave the consequence that too much heat would be removed from thesurroundings of the ringshaped shaped groove and that this surroundingarea would freeze, which is undesirable.

The double cross-sectional surface area of the ring-shaped groove ispreferably greater than the sum of the cross-sectional surface areas ofthe at least one channel. The sum of the cross-sectional surface areasof the at least one inlet channel to the ring-shaped groove preferablyremains constant or is reduced. The sum of the cross-sectional surfaceareas of the at least one channel leading to the cooling placepreferably remains constant or is reduced. By means of such measures,however, the liquid gas is compressed on its path to the place to becooled, so that an optimum cooling effect is obtained.

In order to prevent the occurrence of accumulations of heat,advantageously at those points where an accumulation of heat couldoccur, porous steel is utilized. This material stores the cold and, ifso required, absorbs thermal energy.

The method in accordance with the invention can advantageously be usedfor the cooling of molded parts in injection molding machines withrotating molds. Rotating molds like this provide many advantages. Thereis, e.g., the possibility to inject the molten mass (for example, moltenplastic mass) into the mold from several injection stations. With this,first of all molded parts with different geometrical shapes, differentcolors, or made of different materials can be manufactured. Secondly,with this it is also possible to make molded parts out of severalcomponents (multi-component process). Therefore, molded parts that haveseveral colors or consist of several materials (assembly injectionmolding) can be made. Rotating molds apart from this also make possiblethe utilization of intermediate stations for different operations andshorter cycle times. The liquid gas can be fed in continuously or elsealso in batches, e.g., only then, when cooling within an injectionmolding cycle is necessary.

With the method in accordance with the invention, and with the device inaccordance with the invention, cycle times can be even more massivelyreduced thanks to the exceedingly efficient cooling of just-injectedmolded parts. The invention presented here solves the problem of theintroduction of liquid gas into the shaft rotating from time to time, onwhich the injection mold is suspended.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features of the invention will be apparent withreference to the following description and drawings, wherein:

FIG. 1 is a (p,T) phase diagram for CO₂,

FIG. 2 is a longitudinal section through an exemplary embodiment of adevice in accordance with the invention, used in an injection-moldingmachine.

FIGS. 3-5 is a detail of different embodiments of the device inaccordance with the invention in longitudinal section and

FIG. 6 is a longitudinal section through a further exemplary embodimentof a device in accordance with the invention, used in aninjection-molding machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Used as a cooling medium with the method in accordance with theinvention, and in the device in accordance with the invention, inpreference is CO₂. FIG. 1 shows a (p,T) phase diagram for CO₂, thenumerical values of which have been taken from Landolt-Börnstein,Numerical Values and Functions, volume IV, 4th part, Springer-Verlag,6th printing, 1967, pages 178-179 and 296. In this diagram, on thehorizontal axis the temperature T in ° C. is marked linearly and on thevertical axis logarithmically the pressure P in 10⁵ Pa (whichapproximately corresponds to one atmosphere). A region of the solidphase 91, a region of the liquid phase 92 and a region of the gaseousphase 93 can be differentiated between. These regions 91-93 areseparated from one another by the melting pressure curve 94, a vapourpressure curve 95, resp., a sublimation pressure curve 96. The curves94-96 meet at a triple point P_(T). Further characteristic points in the(p,T) diagram are a critical point P_(K) and a sublimation point P_(s).

The cooling medium CO₂ is preferably brought to the part to be cooled ina compressed liquid condition at temperatures of −50 to −20° C. (223 to253 K). This temperature range 97 in FIG. 1 is indicated with brokenlines. The course of the vapour pressure curve 95 in this temperaturerange 97 shows that high pressures between approx. 7*10⁵ and 20*10⁵ arenecessary in order to keep the CO₂ liquid. If this requirement is to befulfilled, then the cross section of inlet channels on the way to thepart to be cooled must not significantly increase. Also no tight placesmust occur; this makes the transfer of the cooling medium from the fixedobject to the rotating object particularly difficult.

A further possible cooling medium is nitrogen (N₂).

FIG. 2 illustrates a schematic longitudinal section through an exemplaryembodiment of a device in accordance with the invention. The device isbuilt into an injection molding machine with a mold 1 rotating around arotation axis a. Shown of it schematically is only the mold 1, composedof first half mold 11 and a second half mold 12. The first mold half 11has a cutting 13, through which molten plastic mass can be injected intoa forming hollow space 14 between the first mold half 11 and the secondmold half 12. Of an injection nozzle 2 only a part, for example, aheat-conducting torpedo 21 is illustrated. The second mold half 12 ismounted on a shaft 3, for example, a hollow shaft and together with itforms a rotating object 4. The shaft 3, for example, can be rotatablysupported on ball bearings 31.1, 31.2 in a fixed object 5. Such arotating injection mold 1, as mentioned at the beginning, has manyadvantages.

The object now is to efficiently cool with liquid gas a part 15 locatedclose to the forming hollow space 14 of the second mold half 12 and/orthe molten plastic mass, or a molded part created from it by solidifyingand situated in the forming hollow space 14. For this purpose, the part15 to be cooled of the second mold half 12 is equipped with and, in thisexample of an embodiment ring-shaped expansion chamber 65, in whichpressed in liquid gas vaporizes and expands. For the purpose of pressingin the liquid gas, the fixed object 5 has an inlet channel 61 for liquidgas. The rotating object 4 is surrounded by a ring-shaped groove 62,into which the inlet channel 61 merges. The ring-shaped groove islocated on a contact surface 45 between the fixed object 5 and therotating object 4; this contact surface 45 corresponds to thecylindrical external surface of the shaft 3. The transfer point betweenthe inlet channel 61 and the ring-shaped groove 62 can be sealed with aseal implemented as a cutting (not illustrated). The rotating object 4has a channel 63 consisting of, for example, two channel parts 63.1,63.2 for liquid gas, which leads from the ring-shaped groove 62 into theexpansion chamber 65. In the example of FIG. 2, the channel 63 splits-upinto two or also several channel branches 64.1, 64.2. The latterchannels 64.1, 64.2 in the rotating object 4 typically have very smalldiameters of 0.5 mm or less.

In order to enable an expansion of the gas at the desired place 15, theexpansion chamber 65 has a much greater total cross-sectional surfacearea 2A_(E) than the sum A_(K641)+AK₆₄₂ of the cross-sectional surfaceareas of the channel branches 64.1, 64.2; the total cross-sectionalsurface area 2A_(E) of the expansion chamber 65 is in preference somehundred times, for example, 600 times greater than the sumA_(K641)+AK₆₄₂ of the cross-sectional surface areas of the channelbranches 64.1, 64.2. In doing so, these cross-sectional surface areasA_(E), A_(K641), A_(K642) are in essence measured in a plane vertical tothe respective direction of flow of the gas. If the gas has two paths atits disposal, such as, e.g., in the ring-shaped expansion chamber 65,then for the calculation of the total cross-sectional surface area2A_(E) the corresponding cross-sectional surface area AE has to becounted double.

In order to, on the contrary, prevent a vaporisation and/or an expansionof the gas on the way to the expansion chamber 65 and to compress theliquid gas even more for the purpose of achieving an optimum coolingeffect, the inlet channel 61, ring-shaped groove 62, and channels 63.1,63.2, 64.1, 64.2 are dimensioned as follows. The cross-sectionalsectional surface area A_(z) of the inlet channel 61 (measured in aplane vertical or perpendicular to the direction of flow of the liquidgas) is greater than the twice 2A_(N) the cross-sectional surface areaA_(N) of the ring-shaped groove 62 (also measured in a plane vertical orperpendicular to the direction of flow of the liquid gas, i.e., in aplane that contains the rotation axis a). Twice 2A_(N) thecross-sectional surface area A_(N) of the ring-shaped groove 62 isgreater than the cross-sectional surface area A_(K631), A_(K632) of thechannel 63, resp., the sum A_(K641)+AK₆₄₂ of the cross-sectional surfaceareas of the channel branches 64.1, 64.2 reduces towards the expansionchamber 65, for example, at one or more contractions 66.1, 66.2, inpreference each time by 5 to 10%.

In summary, therefore for the cross-sectional surface areas A_(z),A_(N), AK₆₃₁, A_(K632), AK₆₄₁, A_(K642), A_(E) therefore the inequality2A_(E>A) _(z)>2A_(N)≧A_(K631)≧A_(K641)+A₆₄₂ is applicable, whereby thefactor ahead of A_(E) for the example of a ring-shaped expansion chamberamounts to 2, for other geometries, however, can also assume anothervalue, for example, 1.

After the expansion of the gas in the expansion chamber 65, the gas istaken away in a gaseous condition. For this purpose, e.g., the secondmold half 12 in the region of the expansion chamber 65 can at leastpartially be made of porous steel, and the gas can be removed throughthe pores. Alternatively, the second mold half 12 can be equipped withevacuation bores 67, through which the gas is brought to the outside orinto the forming hollow space 14. Such evacuation bores 67 can also beimplemented as expansion bores with great cross-sectional surface area;their total cross-sectional surface area can preferably be some hundredtimes, for example 600 times, greater than the sum A_(K641)+AK₆₄₂ of thecross-sectional surface areas of the channel branches 64.1, 64.2. Theevacuation bores 67 can, for example, be produced by galvanizing. On thesurface of the second mold half 12, the gas removed can either simplyescape to the ambient atmosphere. It can, however, also be collected,liquefied, brought back to a tank and used again for cooling; with arecycling like this, approx. 70-95% of the gas can be re-used after acooling process, which is very efficient.

In the FIGS. 3-5, a detail of different embodiments of the device inaccordance with the invention is depicted in longitudinal section,namely the ring-shaped groove 62, a part of the inlet channel 61 in thefixed object 5 and a part of the channel 63 in the rotating object 4.For the sake of simplicity, in FIGS. 3 and 4 only one half of thelongitudinal section of the rotating object 4 is illustrated. In theexample of an embodiment of FIG. 3, the ring-shape groove 62 is also, asin FIG. 2, machined into the rotating object 4. FIG. 3 shows a variantwith several, for example, two inlet channels 61.1, 61.2. FIG. 4 depictsan example of an embodiment, in which the ring-shaped groove 62 ismachined into the fixed object 5. FIG. 5 concerns a combination of theFIGS. 3 and 4 to the extent that the ring-shaped groove 62 is locatedboth in the rotating object 4 as well as in the fixed object 5.

In the examples of embodiments of the FIGS. 2-5, the liquid gas wasrespectively pressed radially inwards, vertical or perpendicular to theaxis of rotation a, into the ring-shaped groove 63. This, in accordancewith the invention, does not necessarily have to be the case. FIG. 6shows (an otherwise analogous to FIG. 2) an embodiment of the device inaccordance with the invention, in which the liquid gas is pressed into aring-shaped groove 62 parallel to the axis of rotation a. Thering-shaped groove 62 in this example is machined into the second halfmold 12. The contact surface 45 in which the ring-shaped groove 62 islocated in this example of an embodiment is vertical to the axis ofrotation a. Other embodiments are conceivable, in which the liquid gasis even pressed into the ring-shaped groove 62 radially outwards or inanother direction. It goes without saying, that combinations of theembodiments illustrated in the FIGS. 2-6 belong to the invention.

What is claimed is:
 1. A method for internal cooling of a rotatingobject (4) with liquid gas, said rotating object rotating around arotation axis (a), said gas being introduced into said rotating object(4) from a fixed object (5), wherein said liquid gas is pressed from atleast one inlet channel (61) in said fixed object (5) into a ringshapedgroove (62) located on a contact surface (45) between said fixed object(5) and said rotating object (4), pressed from said ring-shaped groove(62) into at least one channel (63, 64.1, 64.2) in said rotating object(4) and delivered to at least one part (15) of said rotating object,said gas, upon reaching said at least one part, evaporating whileabsorbing vaporization heat and thereby cooling said at least one part,and is removed in the form of gaseous gas.
 2. The method in accordancewith claim 1, wherein a greater cross-sectional surface area (A_(E)) ismade available to said liquid gas in an area (65) surrounding said atleast one part of said rotating object (15) than a sum(A_(K641)+A_(K642)) of the cross-sectional surface areas of said atleast one channel (63, 64.1, 64.2), as a result of which said liquid gasexpands and evaporates while absorbing vaporization heat.
 3. The methodin accordance with claim 1, wherein a total cross-sectional surface areaavailable to said liquid gas on its path from said fixed object (5) tosaid at least one part of said rotating object (15) is left constant orreduced, so that said liquid gas does not expand.
 4. The method inaccordance with claim 1, wherein a total cross-sectional surface areaavailable to said liquid gas on its path from said fixed object (5) tosaid at least one part of said rotating object (15) is reduced at leastonce, so that said liquid gas is compressed.
 5. The method in accordancewith claim 1, wherein said liquid gas is pressed into said ring-shapedgroove (62) in the direction of said rotation axis (a).
 6. The method inaccordance with claim 1, wherein said gas is selected from the groupconsisting of carbon dioxide and nitrogen.
 7. Application of the methodin accordance with claim 1, for use in cooling molded parts in injectionmolding machines with rotating molds (1).
 8. A device for internalcooling of a rotating object (4) with liquid gas, said rotating objectrotating around a rotational axis (a), said gas being introduced to saidrotating object from a fixed object (5), said fixed object (5) having atleast one contact surface (45) with said rotating object (4), whereinsaid fixed object (5) has at least one inlet channel (61) for liquidgas, wherein, at said contact surface (45) between said fixed object (5)and said rotating object (4), there is a ring-shaped groove (62) intowhich said at least one inlet channel (61) merges and the center ofwhich is on the axis of rotation (a) of said rotating object (4), andwherein said rotating object (4) has at least one channel (63, 64.1,64.2) for liquid gas, which leads from said ring-shaped groove (62) intoan area (65) surrounding a part (15) of said rotating object.
 9. Thedevice in accordance with claim 8, wherein the area (65) surroundingsaid part (15) of said rotating object has a greater cross-sectionalsurface area (A_(E)) than a sum (A_(K641)+A_(K642)) of thecross-sectional surface areas of said at least one channel (63, 64.1,64.2).
 10. The device in accordance with claim 9, wherein the area (65)surrounding said part (15) of said rotating object comprises at leastone expansion chamber.
 11. The device in accordance with claim 8,wherein a sum of said cross-sectional surface areas (A_(Z)) of said atleast one inlet channel (61) is greater than twice (2A_(N)) across-sectional surface area of said ring-shaped groove (62).
 12. Thedevice in accordance with claim 8, wherein two times (2A_(N)) saidsurface area of said ring-shaped groove (62) is greater than a sum(A_(K631)) of said cross-sectional surface areas of said at least onechannel (63).
 13. The device in accordance with claim 8, wherein a sum(A_(Z)) of said cross-sectional surface areas of said at least one inletchannel (61) to said ring-shaped groove (61) remains constant orreduces.
 14. The device in accordance with claim 8, wherein a sum ofsaid cross-sectional surface areas (A_(K631)) of said at least onechannel (63) toward said part (15) of said rotating object remainsconstant or reduces.
 15. The device in accordance with claim 8, whereinsaid ring-shaped groove (62) is machined into at least one of saidrotating object (4) and said fixed object (5).